This invention relates to an optical printer, such as a laser beam printer, and more particularly to a horizontal sync signal generating device for synchronizing the horizontal scanning starting positions of an optical beam, such as a laser beam.
The fundamental structure of the laser beam printer (hereinafter referred to "LBP") is as illustrated in FIG. 6, wherein a laser beam projected from a semiconductor laser 11 and containing image data, such as graphics and characters, is directed to a rotatably driven polygonal mirror 12 via a collimator lens 16 and a cylindrical lens 17 and is reflected on its reflection planes 12a through 12f. The reflections are radiated onto a photoconductive drum 13. Located between the polygonal rotary mirror 12 and the photoconductive drum 13 is a f.theta. lens 14 which makes constant a horizontal scanning speed of the laser beam on the drum 13. While horizontal scannings are carried out by the reflection planes of the polygonal mirror 12, the photoconductive drum 13 is rotated at a predetermined speed in a vertical scanning direction perpendicular to the horizontal scanning direction. Thus, latent images of entire video signals are formed on the photoconductive drum 13. The latent images are then developed on paper using conventional copy machine technology.
In the LBP as described above, an individual horizontal scanning start position, that is, printing start point of the latent image formed on the photoconductive drum 13, must be synchronized to be brought into alignment with one another in the vertical scanning direction. For this purpose, a photodetector 15 is located on one side of the drum 13 where the laser beam is admitted with rotation of the polygonal mirror 12 without affecting formation of the latent images. Horizontal synchronization can be obtained by setting the scanning start positions on the drum 13 based upon the incidence of the laser beam onto the photodetector 15.
FIG. 7 shows a conventional horizontal sync signal generating device using the above photodetector 15. When the beam enters the photodetector 15 (which comprises a photodiode PD), the electric current ip which depends on the quantity of the received light that flows through transistor TR1 and load resistor RL. The voltage across the load resistor RL is compared with a reference voltage Vref in a comparator C. A horizontal sync signal HL (pulse) is then outputted when the voltage across the load resistor RL exceeds the reference voltage Vref.
Assuming that the reference voltage Vref is constant, timing the generation of the horizontal sync signal is dependent on the magnitude of the voltage across the load resistance RL, that is; the quantity of light admitted into the photodiode PD. Voltage Vx across the load resistance RL is given by the equation : Vx=ip.times.hFE (amplification rate of the transistor TR1) x RL (resistance value of the load resistance) RL.
FIG. 8 which shows the timing the generation of the horizontal sync signal HL in relation with the voltage across the load resistor RL, reveals that the smaller the voltage across the load resistor RL, the more delayed is the generation of the horizontal sync signal HL. The delay time is indicated as .increment.t. This means that, if reflectivity is different among the reflection planes 12a to 12f of the rotary polygon mirror 12, the scanning start positions formed on the photoconductive drum 13 differ at every horizontal scanning and are out of alignment in the vertical scanning direction. In fact, it is difficult to keep the reflectivity on the reflection planes 12a to 12f constant. The difference in reflectivity tends to increase as the operation goes on because of aging, deposition of dust, and etc. Such misalignment in print starting positions results in a poor printing quality, including non-straight vertical lines, which are particularly undersirable when printing graphics.